Apparatus for measuring the magnetic susceptibility of gases



Jan. 19, 1954 Q W, MUNDAY 2,666,893

I APPARATUS F'OR MEASURING THE MAGNETIC SUSCEPTIBILITY OF GASES Filed Deo. 19, 1950 2 Sheets-Sheet l Iyf IN VEN TOR.

C HARLES Wmv-:R MUNDAY.

ATTURNEYJ.

Jan. 19, 1954 c. w. MUNDAY 2,666,893

APPARATUS FOR MEASURING THE MAGNETIC SUSCEPTIBILITY OF GASES Filed Dec. 19, 1950 2 Sheets-Sheet 2 2l-f/' Inv/ ff 7 INVENTOR. CHARLESWALTEE MUNDAY.

BY J /l/,QJQ

ATTORNEYS.

Patented Jan. 19, 1954 UNITED STATES ATENT OFFICE APPARATUS FOR MEASURING THE MAG- NETIC SUSCEPTIBILITY OF GASES Application December 19, 1950, Serial No. 201,551

Claims priority, application Great Britain December 22, 1949 11 Claims.

This invention is for improvements in or relating to apparatus for measuring the magnetic susceptibility of gases, and to determine their chemical components accordingly.

It is known that the magnetic susceptibility of gases can be determined by suspending a solid body in a non-uniform magnetic neld surrounded by the gas under test and measuring the force acting on the body, as is described by Faraday, in Royal Institution Proceedings, vol. 1 (1851), page 254.

The property of magnetic susceptibility of gases can be employed to determine the composition of the gases under test. Thus, for example, if a gas is passed continuously or discontinuously around the test piece, variation in the composition of the gas creating a variation in the magnetic susceptibility of the gas will cause a variation in the position taken in the magnetic field by the suspended body and this movement of the solid body can be employed, when suitably calibrated, to determine changes in the composition of the gases being tested.

There has been described in the art an apparatus for determining changes in the composition of gases wherein the gases are passed through an apparatus, placed in a non-uniform magnetic i-leld, wherein said apparatus contains a test piece freely suspended on a quartz fibre, said fibre carrying a mirror. Deiiection of the test body, corresponding to changes in the composition cf the gas, are observed by means of the mirror and a light spot, as for a mirror galvanometer.

This type of apparatus is not adapted for commercial use in plants for the continuous testing of gases by ordinary plant operatives. The freely suspended test piece is very sensitive to the passage of gases through the apparatus and to mechanical shocks. An important objectof the invention is to provide a practical apparatus which may be easily operated by ordinary plant operatives to determine the components of such gases.

The invention accordingly comprises the novel apparatus and combinations thereof, speciiic embodiments of which are described herein by way of example only and in accordance with the manner in which I now prefer' to practice the invention.

The present invention is for an apparatus for measuring the magnetic susceptibility of gases which comprises a magnet and a cell, preferably constrained to slide between the facing poles of said magnet providing a chamber between the facing poles of said magnet made of a substantially non-magnetic material, having a frame and two magnetic pole pieces shaped to produce a non-uniform magnetic ield tightly ntting in the ends thereof, a test piece suspended in said chamber between the two pole pieces on a fibre rigidly supported at both ends within the chamber, means for the passage of gas through the chamber, said chamber being otherwise substantially gas tight when in position between the poles of the magnet, and means for the measurement of the angular deflection of the test piece in accordance with the magnetic susceptibility of the gas in said chamber.

The present invention also comprises the cell, which is preferably adapted to slide between the facing poles of a magnet, said cell consisting of a frame, open at both ends to be adjacent to the poles of the magnet, made of a substantially non-magnetic material containing preferably slidably therein, and fitting into the ends of the frame, two pole pieces shaped to produce a nonuniform magnetic field and suspended therebetween a test piece, of the kind described, suspended on a bre, said bre being rigidly supported at both ends within the cell, said cell being substantially gas tight when in position between the poles of the magnet and provided with means for the passage of gas therethrough, and means for the measurement of the angular delection of the test piece, such as a mirror carried by the nbre.

The test piece may comprise a solid or hollow body made of a material of substantially zero magnetic susceptibility, or slight diamagnetic volume susceptibility, for example of the order of --0.5 10=-9 to -2.5 l09 c. g. s. units, in any suitable shape, such that when said body is susof low magnetic reluctance such as mu metal."

stalloy and the like. The cell frame may be made of a substantially non-magnetic material such as copper,'brass, dural and the like.

The optimum shape of the pole pieces which will give the desired results, for example, maximum field strength or maximum eld gradient consistent with the minimum volume of the cell may be .calculatedirom known principles. The shape of pole pieces which is preferred is shown in the accompanying drawings.

Very suitably the test piece comprises quartz dumb-bells of the type consisting of two blown quartz bulbs with substantiallyfequalfdimensions and weight connected by a quartzfrod said con-- necting rod being cemented at the centre, and at right angles to the supporting Anbre. Increase in the weight of the quartz dumb-bellsisattended by loss of the sensitivity of the instrument and suitably the quartz dumb-bell'itest piece'shold not weigh more than about milligrams, The lighter the dumb-bells, the more capable gthe cell will be to resist any mechanical shocks and the smaller will be the volume magnetic ssusceptibility of the test piece. The bre must be capable of carrying ethe :dumb-.bells rbut its Vdi- Amensions must :not-ofcourse besuch fthat itis :impossible :to amplify :the ideiiections of the `test piece satisfactorily. The Fmeasurement -of the angular 4deflection of :the `rtest piece is suitably -effected by `cenrienting Va mirror on to the test body, the deiiectionsfoithe test ,piece and ofthe fibre being measured by :means of alight spot .in the lusual way. :A'counterweight maybe pro- `videdso-that the-centrefofinertiaof thetest-piece .substantiallycoincides with theaxis-,of the torsion fibre.

-However ythe cementing -of such countern weight to the test piece lpresents considerable practical dilculties, land @it .is a :further object of the present invention to providea :test piece Vyoftlreftype'described supportedon a=breof the kind described, vwherein fthenecessity of -having iso-employ a counterweightlisgavoided. This can ,suitablybeleffected vb y c'ementing the'dumb-bell type test `piece Aat fits Ycentre to the ,suspending bre andra'tright angles'thereto, fthe'barbetiveen the :bulbs "of the `test piece ,being so curved that the centre roi inertia -o'f the test :piece coincides with 'thev axis o'f thetorsion bre.

When employing va tmirror in 'conjunction with the test piece, the `mirror maybe cemented to the bar joining the 'bulbs of vthe test piece in a position substantially at the-centre of gravity thereof, .and the Ycurvature nf said ,bar being such that the centre of inertia of the test piece and mirror unit ,coincides with the axis ,of the torsion libre.

The degree of .curvature .of .the bar .connecting the bulbs of the .test .piece can :becalculated ,from yknown principles.

Thus, for example, `with aa test piece lcarrying .a;mirror, -thegclegreefof Acurvature vof zthefbar join- `ingthe "bulbs of kvthe :testfpiecerequired:suchithat Vthe centre :of inertia fo'f M'the :test Vpiece rcoineides ywith the `axis of the 1torsion bre, 'is that 'said "bar is `bent 'into the l'segment o'f an 'anchor ring of mean radiusiR, where Ris given approximately by the formula lwhere the dumb-bell consists of va cylindrical rod, of radius "a, length"2b and'inass"c at each kend of which is Aahollow,sphere of radius dand .mass"e, .and the mirronattachedtothe centre ofthe .rod being square nf side .21f,..thickness 2g, .andmassa 4, Thus, for example, Where v -.0.125 mm. b=1.5 mm. 0:1.0imgllL d=l'.35:mm. e=2.25 mgm. f=l.0 mm. 9:0.09 mm. -h=0.5 mgm.

and substituting in the formula above .5R=21.'7 mm.

It is important, if the calibration of the instrufmeritlisftdbesubstantially unaffected by changes in .lev el, ,that the dumb-bell be mounted at its of any Across-section made of :any material of 4suitable elasticity. it :has .lbeen '.foundvery suit- :able to use 4a `quartz :bre -or a goldealloyfsuspension strip 'or 4a :platinum-iridiumfsuspensicn l strip.

'fl/"eryssuitably".thefbre is'carriedbyed supporting ,unit -or member :contained .within :the cell,

A`wherein one end of the iibre is trigidly-xcd 1to "rapidly to 'the composition yof v"the Agases Apassing thrnughztlie Acell,:and consequently 'the ycell should .be capable vof being LAswept outbygas in a 4short time, without, however, causing :any turbulence .in :the f cell which lwould f result :in `the 'mechanical Vdisturbance of the test piece and interfere with 'the satisfactory use of the apparatus.

vIt isfa further fobjectzof thefinventionrto ensure that V,the vpassage of Vgas through the cell can -be 4effected -wit-hout mechanically disturbing-the test gpiece, and'without causing turbulence in'the cell.

rPhis .can suitablyfbefelectedby introducing the :gases into the cell-through a capillary tube .directed away from the position ofthe 1test1piece wherein the capillary tube terminates-inathecell infa position aswremote-fromthe test piece as is possible, and :desirably such lthat the Vincoming .-gasesnimpingezon afsurface-,contained inthe cell :at :rightsanglesto thefdirection pf `flow of the gases. Nery :suitably :bath the ;irilet :and Aoutlet vtubes :are arranged Lin-the same way as Ydescribed above, ,the .inlet and outlet tubes being ldirected fin Vsubstantially `opposite directions. This 'form 4of construction has the ,advantage that inadvertent confusion between the inlet and outlet V'tubes will have no deleterious rresult. 'In this way streamline gas flow can `be obtained gin Ythe cell,and yconsequently `the vcell can'be 'swept yout veryrapidly without disturbingthe test-body. The pressure gradient across-the lcell should =not be large, as otherwise turbulence in the cell cannot b e avoided, and is preferably lin ,the `range .1.0-=3 mm. o f water. Desibiy, Athegas entering fthe lcell should be :under substantially atmosphericupressureeand ithegas dine to the cell vprnvidedwtha relief-valve ofgsuitable design.

In order to describe the invention more clearly, one form of the apparatus according to the present invention is shown, solely by way of illustration, in the accompanying drawings.

Figure 1 shows a plan view of the cell, and Figures 2 and 3 are diagrammatic cross sections along lines AA and BB respectively on Figure 1. Figure 4 is a side elevation showing the detail of the support for the test piece. Figure 4A is a fragmentary front elevation view of an alternative arrangement with a mirror on the test piece.

Figure 5 is a side elevation of an alternative support for the test piece and Figure 6 is an enlarged cross-section along the line CC in Figure 5. Figure 7 shows a side elevation of a bre and associated spring.

The cell comprises the frame made of a substantially non-magnetic material, such as brass, said frame consisting of a three-sided unit I, the fourth side 2 being capable of being secured into said unit, the side 2 being provided with a window 3 made of a transparent material such as glass. This frame contains slidably therein the pole pieces 4 and 5, the faces 6 and I of which mate with the facing poles of the permanent magnet employed. The pole piece 4 carries therein the supporting body 3, suitably made of brass and the base of which ts slidably into the pole piece. To the arms of the body 8 the suspension fibre, suitably a quartz bre 9, is fixed,

and said fibre has cemented at right angles thereto, in the position which will correspond to the gradient of eld strength most appropriate to the application of the apparatus, the test body I0, l

which in the present instance is a quartz dumbbell. The suspension bre 9 also carries a mirror II and a counterweight I2. Alternatively, as shown in Fig. 4A, the mirror may be cemented to the bar joining the bulbs of the test body I0,

and the counterweight I2 may be eliminatech The cell is provided with a gas inlet and outlet tubes I3 and I4 comprising capillary tubes which are directed away from the test piece, and which terminate near to the faces I5 and I6 of the frame.

In Figure 5 the G-shaped supporting body 2| carries the member 22 and the torsion head 23 between which the fibre 24 is suspended. The member 22 ts slidably into a recess in the lower arm of the supporting body 2 I, and can be rigidly ixed therein by means of a grub screw 25. The torsion head 23 is of circular cross-section, the lower part of which fits slidably in a circular recess in the upper arm of the supporting body 2|. The torsion head can be rotated by inserting a pin into one of the holes 26 in the circular head thereof, and turning the torsion head with respect to the supporting unit 2I to the desired extent. The torsion head can be lockedin position by means of the grub screw 21. In this unit both the tension and torsion on the fibre can be readily adjusted, the torsion by means of the torsion head 23, as shown above, and the tension by means of vertical movement of the member 2?. in its containing recess.

The bre 24 is cemented to the torsion head, at point 28, and the lower supporting member 22, as shown in the drawing. When employing a quartz fibre, the torsion head is most suitably constructed of silver, the quartz bre being fused T thereto by means of silver chloride. On the other hand, where a metal strip fibre is employed, the torsion head may be constructed of any sub'- stantially non-magnetic metal, most suitablif fbrass or copper, the metal strip being soldered 'i 6 thereto'. The supporting body 2l is suitably provided with an internally threaded hole, as showny into which a supporting member may be screwed, to facilitate its placing in and removal from the cell.

In order to give the fibre greater resilience and to make it more capable to withstand mechanical shocks, the lower end of the fibre 24 may be cemented to a spring e. g. a quartz rod, which may be bent to form a right-angled spring, which is itself cemented to the member 22, or the iibre may be cemented to a U spring as shown in Figure '7.

Very suitably the dimensions of the parts of the cell are as follows: the external dimensions of the cell are 1.0625 inches by 1.26 inches; the overall dimensions of the pole pieces 4 and 5 are 0.8125 inch by 0.34375 inch by 0.75 inch and 0.8125 inch by 0.34375 inch by 0.375 inch respectively; the inlet and outlet tubes I3 and I4 comprise 0.037 inch internal diameter tubes which terminate 0.03125 inch distance from the faces I5 and I6 of the frame. With this cell it has been found that the sweep out time is in the range 5 to 10 seconds Awhen the difference in pressure between the inlet and outlet gases is 10-3 millimeters of water respectively.

The various elements of the cell must be precision tooled to ensure the exact t of each part so that when the cell is in position between the facing poles of the permanent magnet employed, the cell is substantially gastight. As the pole pieces are contained slidably in the frame, the faces 5 and 6 of the pole pieces can mate with the faces of the poles of the permanent magnet, and thus small errors in the workmanship which might lead to a break in the magnetic flux can be overcome. If desired the cell may be coated with shellac or like material to ensure that it is gas tight.

Suitably, the instrument may be arranged so that the suspension fibre lies along a substantially vertical axis. It is important that the instrument be calibrated with the suspension fibre lying in the same axis as when the instrument is in use.

In the operation of the apparatus, which can suitably be applied to the measurement of the oxygen` content of gases, the cell is positioned between the polesof the permanent magnet, and the gas under test is passed therethrough. The deflections of the test body are suitably measured by means of a light spot, reilected by the mirror carried by the suspension bre, the deflections of which may be measured directly on a scale or are more suitably recorded by means of photoelectric cells and associated electrical recording means. One such system is described for example in my copending application Serial No 201,552. filed December 19, 1950. In systems employing light spots where photoelectric cells are used as measuring elements it is desirable that the effects of light variations be eliminated. This may suitably be effected by using a null balance method. According to one such method the output from the photoelectric cell may vbe amplified and used to operate a servo-mechanism that brings the photoelectric cell to a position such that both sides of the photoelectric cell are equally illuminated, or in other words such that the photoelectric cell or cells are in equilibrium or balanced position. The displacement of the photoelectric cell (measured by suitable electrical -or mechanical means) serves asga measure of the angular .displacement tof .the :test piece. The arse Vof this method 'would rrender the readings cbtained :independent auf :fluctuations :in '.the light source, 'caused for example thy means fluctuation According to another method the output fromhe -photocell 'suitably :amplied :and :operating a servo-mechanism :may `.be l'used to tturn za -.torsion head, as .shown in Figure tto ,maintain .the dumb-bell in "the rsame position so that fboth Ihalves :of the'photoelectric cell :are :equally `illufminated. 'The rotation .of :the torsionJheadrmay be :accomplished by :means of a :suitable worm gear and pinion attached to the torsion head.

The 'present iimrenti'on zncludes, inter f alia, .the testing of 'gases'.using .the apparatus hereinbeore deiined, and .also :chemical vprocesses which 'are characterised rby the v'use o'f said :apparatus for the testing of the reaction -and -fother :gasesrin thacontro'l `of such processes.

'The apparatuszo she-.present inveritinn maybe employed 'for the .analysis .of any .gasmixture which :is susceptible ftc :analysis ,by virtue .of )its magnetic properties. Thus for .instance the apparatus 4.may be iused .for the determination of 'paramagnetic :gases oxygen, nitrogen peroxide, L

nitric oxide chlorine dioxide 'lhe lapparatus nds most general Vuse :in :the estimation A:of .oxy- Ven in gas mixtures in lconcentrations Yranging between =01% :and 1.00%. Naturally infsuch esti- "inations, ff'or [example for 'fthe :oxygen content @of :a l

gas, it is-necessary thatmther paraznagnetic gases suchfasnitric.oxidesand the like are not present, or if present, that their feiect fon vr.the estimation is allowedfor.

The ithi'ckn'ess fof 1.the fibre supporting lthe -test Vpiece is l of Vconsiderable importance l so far :as `the robustnessfo'fftheapparatus:is:concerned Where a quartz fibre is :employed'itzshould ce of 1a fiihiclsF :ness 5in 'the :range 15e-2c :mu dialnet-er, sand where a metal suspension strip, for example a gold-strip,

vis used .it should be cof approximately .the ,following dimensions: :010.03 inch Widthahy-.O inch thickness.

The .deflection nf vthe vtest piece `necessary :for the instrument .to :he adequately sensitive is -not large, and for instance when employing .twin `photoelectric fcellifQr :recording the deflections of the test -piece, ias described :in my-,copending application Serial No. 201,552, tiled-Decemberlil, '1950, Athe sizeof 'the =lbreaemployed within the ranges indicated labove, :should -be:suchthat .when the -gas concentration 1.changes from ,zero :oxygen -concentraticn tc-maximumfoXy-gen concentration, the light spot does not leave .thearea of .the

cathodes of the .twin gphotoelectric cell. ,In .this

.way ymaximum .robustness im' a L.given sensitivity Lisobtained. v.The .length .of .the fibre .ought itc besuch .that .the ,timeo oscillation can be overcome by .suitable electrical means, suchraslby the use of con- .densers It is preferred that the libre is about .10 mm. in length.

The magnet referred to in the specication-and claims may be either a permanent magnet or-an electromagnet. itis preferred, vhowever, to use Aa Apermanent magnet. The iield strength of the magnet should be ashigh'as possible to give maxi mum sensitivity to the apparatus.

I claim:

`l. In an apparatus for'measuring the magnetic susceptibility of Vgases which comprises-l-amag net, ,a cell adapted to beinserted :as 'a unit `vbe- 'tween the facingjnoles mffsaid magnet 'and'provding 'a chamber therebetween, said .cell :being made of Aa substantially `.nonmagnetic material, '7

andlhauing a trarne.and"twomagnetic pole #pieces 'shaped to .produce .a knon-.uniform magnetic Seld fitting .in fthefendsiliereoa test :piece suspended in said chamber :and mounted :between the itwc pole pieces fon a fibre rigidly supported at ,both -ends within the chamber, irneans for .the passage .of gas 4'through 'the'.charnber, :said icham'ber :being otherwise substantially 'gastight Ywhen v4in "pc- :sition betweenithemclescf `themagnet .andzzneans 'for the measurement :of wthe :angular adeflection .of the test piece, :in :accordanceswith the :mag- -neticsusoeptibilityfof the gas Iin said chamber.

2. An :appara-tus fas :in claim .l lwherein said .cell is slidable 'between the :facing poles .of :saidmagnet with said mole :pieces :adjacent the ,'Doles of said magnet.

-3. An rapparatus as 4:in :claiml wherein the test piece is made of a .material lselected 1from .the ,group consisting 4of materials lof .substantially -zero :magnetic susceptibility and materials lof .slight diamagnetic volume susceptibility.

'4. Anapparatus :asin claim .-1 wherein the test piece comprises :a dumb-bell consisting .of two equal spheres rigidly connected by a bar.

5. yAn apparatus as in claim l wherein 'the means for .the :passage lof `gas through lthe chamber comprise two capillary .tubes, lwhose mouths are veach directed away rfrom fthe test piece :and are adjacent surfaces within said chamber.

6. An `apparatus :for measuring the magnetic susceptibility of :gases which comprises 'la magnet and -a cell Aconstrained toslide as ;a ,unit between `the -facing poles of said .-magnet, said -cell lccmprising fa frame made-of :a substantially monmagnetic rmaterial, :andcontainingslidably therein'two poile piecesshaped 13.0 '.Drcduce a loon-uniform magneticeld, 1a :tesi-piece suspended between 4the two pole vpiecespnfa bre rigidly supported 4within the cell, -means for the message .of ses through the cell, said cell `,being substantially gastight v.when :imposition between the lpoles v.of theemagnet, and Kmeans :for the measurement :of 'the :angular qdeiiection of rthe test gpiece.

f7. .An apparatus for .measuring the :magnetic susceptibility of gases which comprises a magnet and :a rcell :rprcyiding :a .chamber zand `slidable 4as :a unit between the facing jpoles -of said magnet .made `olf :a substantially :non-.magnetic material. lhaving :frame and two .magnetic pole lnieces shaped to 'produce f a non-*uniform magnetic field tightly :itting in the .ends thereoffaftest1piece Asuspendedin said-chamber between the =t,w.o pole pieces and 4in lsaid non-uniform magnetic :eld on a fibre frigidlysupported between twolarms .of a :member I.contained within the cell, one'end .Uf the :libre rbeing rigidly xed -to -one varm .of .said member :and .the other aend @of fthe fibre :rigidly .'xed itc :a movable, torsion head fsunpcrted um Vtnegotiierarmof;said'member,:means lfor themas- :sage f of :gas through the chamber, said .chamber 'being otherwise :substantially gastight .when .in position between the :polesfof the magnet, .and means for themeasurement of thefangulardeflectionfof the testpiece, in accordance y.with-.the

magnetic susceptibility of -the,-gas;in.-said chamber from'time totirne.

=8. An apparatus Vfor measuring v.the magnetic susceptibility oflgases which comprisesa .magnet and la cellconstrained to slide|between .the .facing ,poles of said magnet, .said `cell 'comprising a .frame ,made of a .substantially .non-.magnetic material, `and `.ccntaining .therein .two pole vpieces .shaped .tcprcducesainoneunifcrmmagnetic.nekt la test piece suspended between .the .two ,pole

5 #pieces on A,a lfibre -rigidly `Sillportcd within A.the

cell, said test piece comprising a dumb-bell consisting of two substantially identical spheres connected by a bar, said bar being cemented to the fibre, in such a position and being so curved that the centre of inertia of the test piece coincides with the axis of the supporting bre.

9. An apparatus for measuring the magnetic susceptibility of gases which comprises a magnet and a cell constrained to slide as a unit between the facing poles of said magnet, said cell comprising a brass frame and containing therein two mild steel pole pieces shaped to produce a nonuniform magnetic field, a test piece consisting of two substantially identical quartz spheres connected by a bar suspended substantially at its centre of inertia on a quartz fibre rigidly supported at both ends within the cell, means for the passage of gas through the cell, said cell being substantially gastight when in position between the poles of the magnet, and means for 20 the measurement of the angular deection of the test piece.

10. A cell, adapted to be positioned between the facing poles of a magnet, comprising a frame made of a substantially non-magnetic material containing therein, tting into the ends of said frame two pole pieces shaped to produce a non-uniform magnetic field, within said cell, a test piecel suspended in said cell Abetween the two pole pieces on a fibre rigidly supported at both ends within the cell and means for the passage of gas therethrough, the cell being otherwise substantially gastight.

11. A cell, as in claim 10 wherein the pole pieces are contained slidably within the frame of said cell.

CHARLES WALTER MUNDAY.

References Cited in the ile of this patent UNITED STATES PATENTS Number Name Date 2,416,344 Pauling Feb. 25, 1947 2,476,304 Kells et al July 19, 1949 

